622 ANALYSIS OF CAST IRON-CHROMIUM, organic body. For details, the reader is referred to the papers in the Comptes Rendus (vol. lii. pp. 1008 and 1250). 3. The quantity of silicon contained in the iron is ascertained by dissolving the metal in hydrochloric acid and evaporating the solution to dryness, moistening with concentrated hydrochloric acid, then dissolving all the soluble matter in water, and collecting the silica on a filter: from this residue, after the graphite is burned off, the quantity of silicon can be estimated, 100 parts of silica representing 46.66 of silicon. 4. Sulphur, Phosphorus, and Arsenic.-The most accurate mode of estimating these substances consists in deflagrating about 50 grains of the finely-divided iron with about six times its weight of a mixture of 5 parts of pure nitre, and I part of carbonate of potassium, in a crucible of silver, or, still better, of gold.* The phosphorus, sulphur, and arsenic are thus converted into phosphoric, sulphuric, and arsenic anhydrides, which form salts by their action upon the potash; when the fused mass is digested in water they are dissolved, whilst the peroxide of iron remains undissolved. The filtered solution is supersaturated with hydrochloric acid, and the sulphuric acid is thrown down by means of chloride of barium; the excess of barium is removed by adding sulphuric acid, and the arsenic is thrown down by a current of sulphuretted hydrogen. The liquid, filtered from sulphide of arsenic, is now neutralized by ammonia, and on the addition of a few drops of solution of sulphate of magnesium, the phosphoric acid is gradually separated, on standing, as the crystalline double phosphate of magnesium and ammonium. The sesquioxide of iron is dissolved in hydrochloric acid, and a current of sulphuretted hydrogen is transmitted, by which copper would be separated as sulphide; the filtrate is boiled with nitric acid, and the iron separated from manganese, cobalt, or nickel, by means of carbonate of sodium in the manner already described (776). § V. CHROMIUM: Er=52'5; or Cr=26 27. Sp. Gr. 6·81. (780) CHROMIUM is a metal which is but sparingly distributed over the earth. Its most important ore is the chrome ironstone, a compound of protoxide of iron with sesquioxide of chromium (FeO, Er2O), which is generally found massive; but has now and * Any traces of vanadium or of chromium would also be oxidized, and on digesting the mass in water would be dissolved out in the alkaline liquid. then been met with crystallized in regular octohedra like the magnetic oxide of iron, to which it corresponds in composition : it is principally supplied from North America and from Sweden. Occasionally the metal occurs in a higher state of oxidation, in combination with lead, as chromate of lead (PbЄre). Indeed it was in this beautiful mineral that Vauquelin, in the year 1797, first discovered the existence of chromium. To obtain the metal, oxide of chromium is intimately mixed, in fine powder, with charcoal, and made up into a paste with oil; it is then placed in a crucible lined with charcoal, and the cover of the crucible is luted on, after which it is exposed for two hours to the heat of a good wind furnace: an agglomerated mass of metallic appearance is thus obtained. It is not pure chromium, but consists of a combination of carbon with the metal. Chromium obtained by this method is very difficult of fusion; it generally forms a porous mass composed of brilliant grains, which are hard enough to scratch glass. In this state it has a specific gravity of about 5'9, which is no doubt lower than it would be after complete fusion. Deville found, on reducing chromium from pure sesquioxide by means of charcoal from sugar, in quantity insufficient for complete reduction, that the mass underwent partial fusion, but could not be melted into a compact button even at a temperature sufficient to fuse and volatilize platinum. If ignited with the hydrated alkalies, alkaline carbonates, or nitrates, chromium is rapidly converted into chromic acid, which furnishes a chromate by action on the alkaline base. It may, however, be heated to redness in the open air without becoming oxidized, and is not acted on by any acid except hydrofluoric acid. Fremy transmits the vapour of sodium over chromic chloride, which is placed in a porcelain tray, and heated to redness in a porcelain tube: the chromium is obtained in brilliant crystals, which belong to the regular system; they are insoluble in aqua regia. Wöhler reduces chromium from the violet chloride by fusing it with twice its weight of zinc under a flux of the mixed chlorides of potassium and sodium: nitric acid is employed to dissolve the zinc, and the chromium is left as a grey brilliant crystalline powder of sp. gr. 6.81. If chromic chloride be mixed with potassium, and heated in a covered crucible, another modification of chromium may be procured; after washing the residue with water, the metal remains in the form of a dark-grey powder; which assumes a metallic lustre under the burnisher. This pulverulent chromium, if heated in the air, takes fire below redness, and burns brilliantly: it is oxidized with great violence by nitric 624 COMPOUNDS OF CHROMIUM WITH OXYGEN. acid, sometimes becoming incandescent during the action; and it is dissolved by hydrochloric acid and diluted sulphuric acid, with evolution of hydrogen. Metallic chromium has not been applied in the arts, but its sesquioxide and many of the chromates are highly valued as colouring materials, both in painting on porcelain and in calicoprinting. (781) COMPOUNDS OF CHROMIUM WITH OXYGEN.-Chromium forms four well-marked oxides: a protoxide, Ere, and a sesquioxide, Er,,, both capable of forming salts with acids; an intermediate oxide (Єre,Єr,,), corresponding to the magnetic oxide of iron; and a stable anhydride (Єrg) which by its action on bases forms salts corresponding to the manganates and ferrates. It also appears to be probable that a perchromic acid (H¤r¤) exists; at least, a blue liquid, which is soluble in ether, is obtained on pouring peroxide of hydrogen into a solution of chromic acid; but none of its compounds are known. Protoxide of Chromium, or Chromous Oxide (Er0=68'5, or Cro=34°3).-This oxide is known only in the hydrated condition. It is obtained as a dark-brown precipitate on adding caustic potash to a solution of the chromous chloride; it absorbs oxygen with great avidity, and even decomposes water with extrication of hydrogen, and then becomes converted into the intermediate hydrated oxide (Єr,Єr,,,H,), which is of the colour of Spanish snuff. 3 The protoxide of chromium forms a double sulphate with sulphate of potassium (Er'K, 2 SO6H), corresponding to the double sulphate of iron and potassium both in form and composi tion. The crystals are of a fine blue colour. (782) Sesquioxide of Chromium, or Chromic Oxide (Er ̧Ð ̧=153, or Cr2O=76·5); Sp.Gr. cryst.5°21: Comp. in 100 parts, Er,68.68; →, 313.-This oxide is obtained as a greyish-green hydrate, by boiling with alcohol a solution of anhydrochromate (bichromate) of potassium acidulated with sulphuric acid. The alcohol deprives the chromic acid of half its oxygen, and the liquid becomes green from the formation of chromic sulphate. A current of sulphurous acid may be employed instead of alcohol in the reduction of chromic acid. On the addition of ammonia, a bulky, gelatinous, bluishgreen precipitate of the hydrated oxide is produced, which retains alkali with great obstinacy even when boiled with water. In this form it is freely soluble in acids, and forms salts, the solutions of which are of a green colour; but they do not crystallize. (See note, next page.) SESQUIOXIDE OF CHROMIUM. 2 625 Sesquioxide of chromium gives rise also to another set of soluble salts, which are of a violet colour and crystallize readily. If these violet-coloured salts be precipitated by ammonia, they give a bluish-green hydrated oxide, which if redissolved in an acid without the application of heat, reproduces the violet-coloured salts. This precipitated oxide becomes green by the action of concentrated saline solutions upon it. If a solution of one of the violet salts be heated to the boiling-point, or a little short of it, the salt at once becomes green.* The hydrated oxide from the violet salts is the metachromic oxide of Fremy (Er,,, 9 H2, when dried in vacuo), and is soluble in excess of ammonia in presence of acetic acid: but according to Siewert, who has lately examined this precipitate with great care, it is not a pure oxide, but a compound of the hydrated oxide with ammonia and one of the salts of ammonium. It is soluble in excess of ammonia and acetic acid, owing to the strong tendency of chromic oxide and ammonia to produce, in the presence of acetic acid, double salts, which are not susceptible of decomposition by ammonia. If the original precipitate be thoroughly washed till all the soluble salts are removed from it, and then air-dried, it is no longer soluble in acetic acid, or in other diluted acids. 2 The only way to ensure the production of a pure hydrated chromic oxide, according to Siewert, consists in precipitating a soluble chromic salt by the addition of ammonia, and boiling it with excess of the alkali. A light-blue precipitate is thus obtained, which, when well washed and dried in air over oil of vitriol, consists of (Єr ̧Ð ̧, 7 H2O). If dried in vacuo, it retains only 4 H2, and if dried in a current of hydrogen at 400° it retains H2O; it then forms a blue powder with a greenish lustre, and Ꮋ Ꮎ is insoluble in boiling diluted hydrochloric acid. Hydrates of potash and soda precipitate the hydrated sesquioxide, and redissolve it if added in excess, forming a green solution, from which, on boiling, the whole of the sesquioxide of chromium is separated 2 * This change of colour from violet to green has been long known, and was accounted for by Berzelius upon the admission of two distinct hydrates of chromic oxide, one green, the other violet. But Siewert appears to have distinctly proved that one form of the oxide, the blue hydrate, only exists. The green precipitate in every instance is a compound of the pure oxide with potash or soda. It is true that the violet solutions of the pure salts become green by boiling, but this Siewert has traced to the conversion of the normal salt into two soluble salts, each green, and capable of co-existing in the liquid. One of these salts contains excess of acid, the other excess of base (Liebig's Ann. cxxvi. 86). Solutions of the green salts at once become blue or violet, if acidulated by the addition of nitric acid. The investigation of these compounds is beset with unusual difficulties. 626 SESQUIOXIDE OF CHROMIUM. as a green hydrate, which retains a portion of alkali. Indeed potash and soda have so strong a tendency to combine with chromic oxide that if either of these alkalies be used to precipitate the oxide from its salts, or if salts of potash or soda be present when ammonia is employed as the precipitant, the green precipitate always contains one of the fixed alkalies. When the hydrated oxide is heated, it parts with its water below redness, and if heated a little beyond this point, it suddenly becomes incandescent, shrinks considerably in bulk, and is no longer attacked by acids. Besides the soluble variety of the salts of chromium, an anhydrous, insoluble series is known, corresponding, it would seem, to the dense and comparatively inert modification of the metal itself. Anhydrous green oxide of chromium is not decomposed by heat, and hence is used as a green colour in enamel painting. It is usually prepared for this purpose by decomposing basic mercurous chromate by a red heat: half the oxygen of the anhydride is expelled along with the oxide of mercury. Chromate of ammonium may be employed instead of chromate of mercury with equally good results. Another method consists in strongly igniting, in a covered crucible, an intimate mixture of 4 parts of powdered anhydro-chromate of potassium and I part of starch; the carbonate of potassium resulting from the decomposition is washed out, and the sesquioxide of chromium which remains, after undergoing a second calcination, yields a beautiful clear green colour. There are also a variety of other modes of obtaining it. Sesquioxide of chromium is the colouring ingredient in greenstone, in the emerald, in pyrope, and in several other minerals. The pink colour used on earthenware is prepared by heating to redness a mixture of 30 parts of peroxide of tin, 10 of chalk, and I part of chromate of potassium; the product is then finely powdered, and washed with weak hydrochloric acid; a beautiful rose-tint is thus obtained. A beautiful green pigment known as vert de Guignet is manufactured on a large scale by calcining anhydro-chromate of potassium with 3 times its weight of crystallized boracic acid: oxygen and water are expelled, and on washing the residue with water a basic borate of chromium is left, while boracic acid and borate of potassium are dissolved out. 2 3 (783) Chrome iron-stone (FeÐ,Єr‚Ð ̧; Sp. Gr. 45) is the principal ore of chromium: it corresponds in composition to the brown oxide of chromium and to the magnetic oxide of iron; part |